i would not worry about airex if you can source it.
the fact that it is not a high temp foam should not be on your short list of requirements , unless you plan on using prepregs that require elevated temps for curing.
dont use it for engine enclosures and dont let it bake in your car on a hot day.
core-cell does seem easier to find.
jamestown distributers in the NE
fiberglass supply in the NW
http://www.fiberglasssupply.com/Product ... talog.html
fiberglast supply (somewhere else for composite materials)
but also has lots of tutorials regarding construction techniques and materials
a different twist , but my all time favorite construction thread:
http://www.swaylocks.com/forum/gforum.c ... t=16269854